a. Use M/S wherever you need it. It's rare that it's needed everywhere. I have an EQ that does M/S internally (Weiss) and a compressor as well (Manley). Aside from that it's easy to set something up in the DAW as needed.

b. Again, choose what process needs to be M/S, put it there. I don't see how analog M/S will improve the signal quality or sound. It's just a different way to manipulate 2 channels.

c. Digitally when I want to do something ITB, analog when I'm working outside the box. Recalls happen less and less frequently as your skills improve ;-)

d. I don't know of this unit, but if it's based on their 120v mastering series, the other things I've heard have been excellent.

Pre DAC: the Z-Sys digital EQ also includes an M/S section for adjusting levels and EQs.

Analog Hardware: the Rupert Neve Master Bus Processor has a nice SFE (M/S) section for adjusting "width" and "depth" with level, EQ, or compression. It comes very last in my analog chain just before the ADC which feeds the "catching" DAW, Sonic soundBlade HD.

Today I applied a couple clicks of "width" level and/or HF EQ to a great "Americana" record with the Rupert MBP.

Which end of the chain I use M/S with depends on the application, if it's a repair/surgery on a questionable mix, I might use M/S ITB. If it's a simple "enhancement" I'll probably use the Neve.